1. Field of the Invention
The present invention relates to an optical disc composed of two elementary discs adhesively bonded one to the other, each elementary disc containing at least one transparent substrate with an information face provided with surface micro-alterations distributed over a spiral track or concentric tracks and intended to be read optically with the aid of a laser beam.
2. Related Art
Currently, a new standard of high-density optical disc is being devised under the name DVD (Digital Video Disc). A specifically developed technique enables this new type of optical disc to have a storage capacity of greater than 4.7 Giga Bytes per information face, thus making it possible to store over two hours of digital video compressed according to the MPEG2 Video standard. Applications in the field of information processing are also made possible by the high density of the optical disc.
The increase in the density of storage on optical disc has necessitated a change of technique as compared with the compact disc (CD) technology, especially as regards the size of the micro-alterations on the disc, the wavelength of the reading laser, the system of modulation and the method of error correction. As a result of this, a reader of the DVD disc does not a priori allow the reading of conventional compact discs.
The compact disc is around 1.2 mm thick and consists of a transparent substrate including an information face provided with micro-alterations in the form of micro-pits or micro-ridges distributed over a spiral track. The information face is covered with a reflecting metallic layer which is covered in turn with a protective layer on the outer face of which is then affixed a label indicating the contents of the disc. The DVD disc is composed of two elementary discs each around 0.6 mm thick. The two elementary discs are adhesively bonded back to back, on the side of their respective information face if present, by a fine layer of adhesive, this resulting in a disc around 1.2 mm thick. For optical reading, the thickness of the substrate traversed by the laser beam is 1.2 mm and 0.6 mm respectively for the compact disc and for the DVD disc.
The compact disc technique uses EFM modulation, the CIRC error correcting code and the reading wavelength xcex3=780 nm, whereas the DVD technique uses {fraction (8/16)} modulation, the RSPC error correcting code and the reading wavelength xcex3=650 nm or 635 nm. Thus, a reader designed uniquely for the DVD disc is not compatible in respect of the reading of compact discs. To ensure upward compatibility enabling the DVD reader to read compact discs, an electronic unit and a laser source which are dedicated to the reading of compact discs have to be included in the DVD reader, this substantially increasing the price of such readers. Furthermore, although the upward compatibility of DVD readers enables compact discs to be read, such is not the case for the readers of conventional compact discs which cannot in any event read DVD discs since such readers do not include an electronic unit and a laser which are specific to the reading of DVD discs.
The objective of the present invention is to solve this problem of compatibility in respect to the reading of compact discs and DVD discs while proposing a simple and economical solution.
The invention proposes a novel type of optical disc which combines the advantages of compact discs and DVD discs and can be read equally by compact disc readers and DVD readers.
The invention also proposes a method of manufacturing this novel type of optical disc.
The optical disc according to the invention is composed of two elementary discs assembled by adhesive bonding of one to the other, each elementary disc containing a transparent substrate with at least one information face provided with surface micro-alterations distributed over a spiral track or concentric tracks and intended to be read with the aid of a laser beam. The micro-alterations may be in the form of micro-pits, micro-ridges and/or continuous grooves. The information face of the first elementary disc is coated with a semi-reflecting layer adjacent to the assembling adhesive layer which is transparent. The information face of the second elementary disc is coated with a reflecting metallic layer and is situated opposite the assembling adhesive layer. A protective layer subsequently covers the reflecting metallic layer of the second disc so as to serve to support a label for the disc. The semi-reflecting layer has a coefficient of transmission greater than or equal to a first predetermined value for a wavelength of the laser greater than or equal to a first threshold. The semi-reflecting layer furthermore has a coefficient of reflection greater than or equal to a second pre-determined value for a wavelength of the laser less than or equal to a second threshold. Preferably, the first threshold is greater than the second threshold by at least 100 nm.
Advantageously, the information face of the first elementary disc and the information face of the second elementary disc comply with two different standards, for example DVD or CD.
Preferably, the first predetermined value for the coefficient of transmission is equal to 95%. The second predetermined value for the coefficient of reflection is equal to 25%. For the application of the optical disc within the framework of CD and DVD compatibility, the wavelengths used are of the order of 780 nm for the first threshold and of the order of 650 nm for the second threshold, and the overall thickness of the disc is 1.2 mm.
A secondary information face of the second elementary disc, adjacent to the assembling adhesive layer, can be produced on the face opposite the reflecting metallic layer. The secondary information face is coated with a semi-transparent layer which has similar characteristics to the semi-reflecting layer of the first elementary disc versus the wavelength of the laser, and must not substantially impair normal reading of the semi-reflecting layer and of the reflecting layer.